The present invention relates, in general, to semiconductor circuits and, more particularly, to Radio Frequency (RF) circuits.
The performance of an RF power amplifier depends on the impedance of a load coupled to the output of the RF power amplifier. An RF power amplifier is generally designed to perform optimally when the load impedance has a predetermined value such as, for example, 50 ohms (xcexa9). When the load impedance differs from the predetermined value, the performance, such as output power, efficiency, linearity, etc., of the power amplifier is degraded.
In communication applications, an RF power amplifier is typically used to amplify an RF signal before the RF signal is transmitted through an antenna. The impedance of the antenna depends on the environment in which the antenna operates. In order to maintain an optimum performance of the power amplifier in various environments, an isolator is coupled between the power amplifier and the antenna. The isolator, which is also referred to as a circulator, has a first terminal coupled to the output of the power amplifier, a second terminal coupled to the antenna, and a third terminal coupled to ground via a device having a fixed impedance, e.g., 50 xcexa9. The output signal of the power amplifier is transmitted to the antenna through the first and second terminals of the isolator. The signal reflected back from the antenna due to an impedance mismatch is transmitted to ground via the third terminal of the isolator and the fixed impedance device. Thus, the impedance mismatch of the antenna does not affect the performance of the power amplifier. However, an isolator is big, expensive, and power inefficient. It is not suitable for use in low cost, low power, portable communication systems.
Accordingly, it would be advantageous to have a circuit and a method for maintaining the optimum performance of a power amplifier under various operating conditions. It is desirable for the circuit to be simple and inexpensive. It is also desirable for the circuit to be small. It would be of further advantage for the method to be power efficient and suitable for use in low power applications.